The present invention relates generally to fiber optic systems, and more particularly to performance monitors that measure timing-dependent characteristics of the fiber optic signal being monitored.
With increasing demand for telecommunications bandwidth, there has arisen a need for performance monitors that assess the quality of an optical signal at an intermediate or an end point of an optical link. This information may be used during link construction in order to aid in troubleshooting, or while the link is functioning in order to aid in dynamic allocation of optical bandwidth. For example, this information is useful in locating a fault in a link or determining when a signal needs to be regenerated.
The optical signal is typically monitored by tapping a fraction of the optical power from the optical signal channel and converting the tapped optical signal into an electrical signal using a photodiode or other suitable optical-to-electrical conversion device. The actual monitoring circuitry is electrical. The electrical signal that represents the optical power in the optical signal is sometimes referred to informally as the optical signal.
Complete signal monitoring includes both timing-independent (DC) information such as signal power level, noise floor level (signal-to-noise ratio (SNR)), and wavelength drift from the International Telecommunications Union (ITU) grid specified wavelength, as well as timing-dependent (AC) information such as jitter, signal extinction ratio (the ratio of the optical power in a logical xe2x80x9c1xe2x80x9d to the optical power in a logical xe2x80x9c0xe2x80x9d), and bit error ratio (BER).
One approach is described in S. Ohteru and N. Takachio, xe2x80x9cOptical Signal Quality Monitor Using Direct Q-Factor Measurementxe2x80x9d, IEEE-PTL vol. 11 no. 10 at 1307-1309 (1999). This article describes a method of deriving the standard deviation and the mean value of the marks/spaces rail of an eye pattern (diagram) and calculates the Q-factor.
In a traditional receiver, an incoming optical signal is split into first and second signals, and a clock signal is recovered from the first signal. The phase of the recovered clock signal is adjusted to the center of the bit, and the recovered clock signal is used to sample the second signal. A decision circuit determines whether the sampled voltage level corresponds to a logical 1 or 0. The quality of the optical signal can be visually determined by using an oscilloscope, triggered by the recovered clock signal. An oscilloscope trace of the measured optical power for an extended random pattern of logical 0""s and 1""s is referred to as an eye diagram, so-called because it looks like an eye. The eye diagram provides a visual indication as to whether 0""s and 1""s are being adequately differentiated so that the transitions can be adequately detected.
An ideal eye diagram has an open central area bounded by four features. The central area is bounded on the top by a trace of the 1 signal level and on the bottom by a trace of the 0 signal level. On the left, the central area is bounded by a trace of the signal transitioning from 0 to 1 and by a trace of the signal transitioning from 1 to 0. On the right, the central area is bounded by a trace of the signal transitioning from 1 to 0 and by a trace of the signal transitioning from 0 to 1. For real-world signals, factors such as overshoot, rise time and fall time will distort the symmetry of the eye diagram. A high-quality signal exhibits a xe2x80x9ccleanxe2x80x9d or xe2x80x9copenxe2x80x9d eye, with the samples being close together and forming well-defined traces in the eye diagram. A low-quality signal exhibits a xe2x80x9cclosedxe2x80x9d eye, with the samples infringing into the central part of the eye.
A traditional signal monitor may use an analog-to-digital converter (ADC) in place of the decision circuit for digitally monitoring the quality of the optical signal. (Such a monitor is similar to a digital oscilloscope.) However, the bit rate of typical optical signals is in the gigabit-per-second range, and conventional electronic circuitry is not fast enough to take a large number (say 100) of samples of each bit. Instead, the signal is sampled (by clocking the ADC) at a much slower rate (at various points relative to the clock edge) and the sample values are stored in memory. By using a phase adjustment circuit to adjust the phase of the sampling clock between one edge of the bit and the other, a complete sample of the incoming waveform is constructed over time. The samples extracted by the performance monitor may be graphed to form an eye diagram.
The accurate recovery of the clock information from the optical signal is important; without an accurately recovered clock, it is not known whether a given sample occurs at the midpoint of the eye or near the edge.
Without the recovered clock, it is possible to determine the AC statistics of the 1""s and 0""s of the eye; however, this can be translated into eye quality only in the case of well-behaved signals. In the real world, signals are not well-behaved. A good eye and a bad eye can both have the same AC statistics for the ones and zeros. Thus, for real-world signals, eye quality complete with phase information is an important concern.
There is a therefore a need to economically provide full AC quality monitoring information.
In order to overcome the above-noted deficiencies, an AC performance monitoring device and method uses a locally-generated clock instead of recovering the clock from the optical signal. A number of samples are extracted from the optical signal at a timing determined by the locally-generated clock, and are placed in an eye diagram. Numerous eye diagrams are generated using various estimates of the clock rate of the optical signal (i.e., the bit rate). The actual bit rate of the optical signal corresponds to the estimated bit rate having the best eye diagram.
One advantage of the present invention is that the full data stream does not need to be regenerated in order to extract the monitoring information. Rather, the bit rate of the optical signal is determined over many bit times without having to recover the clock signal. This relaxes the need for real-time signal processing.